I had considered making the LED permanent; however, it should have its own DC supply and there's not one handy (unless I include a significant dropping resistor from the +100V rail). I think I'm going to try replacing the neon first and if that doesn't work, then the transistor.
Thanks guys,
Barry - N4BUQ
toggle quoted message
Show quoted text
Or both of us are right and both lamp and transistor are weak.
Alternatively, if he got it to work with a LED instead of the neon, with the
existing transistor, then swap out the neon for the LED circuit.
On 4/15/2024 6:17 PM, Harvey White via groups.io wrote:
Well, one of us has to be right.? Lamp first, transistor second, bet one of
them solves it.
Harvey
On 4/15/2024 6:14 PM, Peter Gottlieb wrote:
If you are seeing 95 volts across the neon lamp and it isn't firing it is bad
and needs to be replaced. You will see exactly this as they age.? It is
normal, once it fires, to have a lower voltage across it.
Peter
On 4/15/2024 6:09 PM, n4buq via groups.io wrote:
I was able to get clips across DS101 and monitor its voltage.? When the
trigger circuit turns it off, there is 28V across the neon which makes
sense.? When the trigger circuit attempts to turn it on, there is 95V across
it but it still isn't illuminated.? That indicates to me the lamp isn't firing.
If I place a 1k across Q109's collector and emitter and turn the scope on,
the bulb illuminates and the voltage across it drops to around 85V which I
presume is normal.
I just don't see why, if the lamp is seeing sufficient striking voltage, the
circuit can't cause it to illuminate. Perhaps Q109 just can't (or doesn't)
pass enough current?
Thanks,
Barry - N4BUQ
Voltage on a neon lamp rises from zero until the strike voltage is
reached (say, 85 to 90 volts depending on lamp and gas). From that
point, the ionized neon has a voltage drop that's proportional to the
current.? It's a negative resistance curve, so the more current the
lower the resistance and the more current.... That's why there's a
current limiting resistor in there.? Once illuminated, if the current
drops below the sustaining current (IIRC) or voltage drops below the
sustaining voltage, then the lamp goes out.
It turns on next when the voltage exceeds the strike voltage.
Different lamps have different strike voltages depending on the gas and
pressure (IIRC).? Radioactive materials can be used to lower the strike
voltage.? Strike voltages also vary with illumination (try this with a
flickering neon and either a laser or a flashlight).
Harvey
On 4/15/2024 4:03 PM, n4buq wrote:
Jeremy,
Presuming you see -8.8V (per the schematic), that pulse is running
between -8.8V and -8.82V, correct?? I remember seeing a pulsed voltage
but I don't recall the exact values and I need to check that again.
In the meantime, I decided to place one of my NOS A9A neons between
the point where the wire coming from the bottom of R136/DS101 and the
junction of R137/Q139's collector (that wire is on a plugin terminal
on the main board near Q109). The A9A will turn on and off as
expected; however, it does not turn completely off and the difference
between the two levels is not very much but it can be distinguished
easily enough. Essentially "off" is not fully off and "on" is not
fully on. I presume that's not what you see, correct?
Thanks,
Barry - N4BUQ
???? At the base of Q109 appears a pulse triggered by the sweep. The
???? pulse is about -20 mV in height and 35 mSec width. This pulse does
???? not appear when in manual mode and the button is pressed. In that
???? case, there¡¯s some kind of event at the end of the sweep but it
???? isn¡¯t a clearly-defined pulse, as seen on my HP-132A ¡®scope.
???? Jeremy
???? On Mon, Apr 15, 2024 at 10:21?AM n4buq via groups.io
???? <> <n4buq@...> wrote:
???????? Jeremy,
???????? I'll check it again but I believe that Schmitt Trigger is
???????? functioning correctly.
???????? I checked the series resistance of R135+R136 and that shows as
???????? 210k so I think both of those resistors are likely good and in
???????? tolerance.
???????? I retried my experiment with another neon but this time I used
???????? a lower value series resistor (27k as opposed to 47k) and the
???????? A9A neon appears to work that way.? It glows rather dimly when
???????? Q109 is "off" but does glow brighter when Q109 is "on".? I
???????? think it glows when Q109 is off because I don't have a
???????? parallel resistor across the neon like the original R136.? I
???????? may try to find a near equivalent to R136 and give that a
???????? try.? If that works, then it looks like the old neon might
???????? just not be getting sufficient firing voltage for its age.
???????? Progress...
???????? Thanks again,
???????? Barry - N4BUQ
------------------------------------------------------------------------
???????????? *From: *"Jeremy Nichols" <jn6wfo@...>
???????????? *To: *"HP-Agilent-Keysight-equipment"
<[email protected]>
???????????? *Sent: *Monday, April 15, 2024 11:59:51 AM
???????????? *Subject: *Re: [HP-Agilent-Keysight-equipment] More
???????????? Investigation of the Malfunctioning Time Base RESET Lamp
???????????? in my HP-1421A.
???????????? Barry, the comment by Harvey is valid: if you can turn on
???????????? DS101 by applying a ¡°signal¡± to the base of Q109, that
???????????? whole circuit is probably OK. You might want to look at
???????????? the Schmitt Trigger circuit (Q110-Q111). It must be
???????????? working if the ¡®scope sweeps normally, but something is
???????????? preventing the base of Q109 from getting the signal it
???????????? needs to turn on DS101.
???????????? Jeremy
???????????? On Sun, Apr 14, 2024 at 6:04?PM Harvey White via groups.io
???????????? <> <madyn@...> wrote:
???????????????? On 4/14/2024 8:59 PM, n4buq wrote:
???????????????? > (New thread due to very incorrect subject line and
???????????????? opening paragraph
???????????????? > in previous thread.)
???????????????? >
???????????????? > I still have not determined why DS101 will not turn
???????????????? on in my HP-1421A
???????????????? > time-base.? I can ground Q109's base through a
???????????????? resistor and force the
???????????????? > lamp to turn on.? I can place a 1K resistor in
???????????????? parallel with R137 and
???????????????? > also force the lamp to turn on; however, nothing
???????????????? else works. I thought
???????????????? > that the neon might have aged to the point where
???????????????? there isn't
???????????????? > sufficient striking voltage for it but since it will
???????????????? turn on with
???????????????? > these methods, that doesn't seem to be case(?).
???????????????? >
???????????????? If you can turn it on, it's not bad.? If you can
???????????????? provide a signal to the
???????????????? transistor that turns it on, it's likely not bad.
???????????????? Where does the signal come from, is it there when it
???????????????? is supposed to be?
???????????????? Harvey
???????????????? > Setting the unit up as described in the manual for
???????????????? voltage tests, I
???????????????? > get the following:
???????????????? >
???????????????? > -100V F(B): -99.9V
???????????????? >
???????????????? > Q109
???????????????? > Emitter: -9.35V
???????????????? > Base: -8.78V
???????????????? > Collector: -0.33V
???????????????? >
???????????????? > Q110
???????????????? > Emitter: -8.01V
???????????????? > Base: -8.11V
???????????????? > Collector: -3.7V
???????????????? >
???????????????? > Given those values, it would appear (at least to me)
???????????????? that the lamp
???????????????? > should turn on but it doesn't.
???????????????? >
???????????????? > As a test, I disconnected R136/DS101 (the yellow (4)
???????????????? wire) from the
???????????????? > board and connected a battery, resistor, and LED
???????????????? between GND and the
???????????????? > collector of Q109.? This works just as it should
???????????????? (i.e. the LED turns
???????????????? > on when the sweep is triggered, turns off at the end
???????????????? of the sweep, and
???????????????? > repeats that).
???????????????? >
???????????????? > Is it possible that Q109 is not able to conduct
???????????????? enough current to turn
???????????????? > DS101 on but enough to make the LED work?? That
???????????????? seems unlikely but, at
???????????????? > this point, I don't have any other explanation.
???????????????? >
???????????????? > Any help with this is appreciated.
???????????????? >
???????????????? > Thanks,
???????????????? > Barry - N4BUQ
???????????????? >
???????????????? >
|
